The production of paper pulp or the like utilizing a continuous digester is susceptible to a wide variety of conditions. In many situations, when the chips are steamed and fed from a high pressure feeder to the continuous digester, a chip meter operates continuously at a constant speed. However it has been found that such an operation is far from optimal, and--depending upon the mass flow to the digester--can cause a greatly varying production rate of pulp from comminuted cellulosic fibrous material.
According to the present invention, it has been determined that by controlling the mass flow to a continuous digester production can be increased and a number of other advantages achieved. According to the present invention, the mass flow of the chips--or other comminuted cellulosic fibrous material--to the digester is controlled by controlling the chips meter that feeds the chips from a source ultimately to the continuous digester. Utilizing the invention it is possible to achieve a significant increase in production while using the same white liquor charge rate. Also, more stable Kappa numbers and digester level control can be achieved.
According to one aspect of the present invention a method of controlling the feed of comminuted cellulosic fibrous material to a continuous digester utilizing a material meter of variable speed is provided. The method comprises the steps of: (a) Substantially continuously feeding comminuted cellulosic fibrous material from the material meter to the continuous digester. (b) During the practice of step (a), substantially continuously determining a quality related to mass flow of the material being fed. And, (c) substantially continuously controlling the speed of the material meter in response to step (b) to provide a target mass flow of material fed from the meter to the continuous digester. Preferably, the quality to be determined that is related (e.g. proportional) to mass flow is the density of the material being fed. In order to more simply determine the density, it is determined in a point in the sequence where the material is not immersed in liquid, but is essentially free falling. In a conventional system this density determination is practiced at the conventional chip chute, above the level of liquid in the chip chute. A liquid level sensor for sensing an overfill condition also is preferably provided at the chip chute, and according to the method of the present invention is shielded from the density detecting equipment, which preferably comprises a nuclear type density gauge. Step (c) is preferably practiced to control the material meter speed so that it is in the range of about 0 to 16 rpm.
According to another aspect of the present invention a system for processing comminuted cellulosic fibrous material is provided. The system comprises: A conduit for conveying comminuted cellulosic fibrous material. A material meter connected to the conduit for metering the flow of material to the conduit. A continuous digester. Means for substantially continuously transferring material from the conduit to the digester. Means for determining the density of the material flowing in the conduit. And, means for controlling the meter in response to the density determining means to provide a target mass flow of material fed from the meter to the digester. Preferably the conduit comprises a chute with a steaming vessel connected to the chute opposite the meter. The density determining means comprises a nuclear type density gauge having a source and a detector mounted opposite each other to the chute. A liquid level sensor, with a shielding means between the source and the liquid level sensor, may be provided.
According to yet another aspect of the present invention a chip chute is provided. The chip chute comprises: Means at the top and bottom of the chip chute for respectively connecting the chip chute to other devices for acting on the chips. A nuclear type density gauge comprising a source and a detector mounted on opposite sides of the chute, to the chute. A liquid level sensor associated with the chute. And, shielding means between the source and the level sensor.
It is the primary object of the present invention to provide for the effective control of cellulosic fibrous material fed to a continuous digester in order to maximize production and obtain other advantages. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.